Thread Confinement

So when an object is confined to a thread, no other thread can have access to it?

No, it's the other way around: if you ensure that no other thread has access to an object, then that object is said to be confined to a single thread.

There's no language- or JVM-level mechanism that confines an object to a single thread. You simply have to ensure that no reference to the object escapes to a place that could be accessed by another thread. There are tools that help avoid leaking references, such as the ThreadLocal class, but nothing that ensures that no reference is leaked anywhere.

For example: if the only reference to an object is from a local variable, then the object is definitely confined to a single thread, as other threads can never access local variables.

Similarly, if the only reference to an object is from another object that has already been proven to be confined to a single thread, then that first object is confined to the same thread.

Ad Edit: In practice you can have an object that's only accessed by a single thread at a time during its lifetime, but for which that single thread changes (a JDBC Connection object from a connection pool is a good example).

Proving that such an object is only ever accessed by a single thread is much harder than proving it for an object that's confined to a single thread during its entire life, however.

And in my opinion those objects are never really "confined to a single thread" (which would imply a strong guarantee), but could be said to "be used by a single thread at a time only".

The most obvious example is use of thread local storage. See the example below:

class SomeClass {    // This map needs to be thread-safe    private static final Map<Thread,UnsafeStuff> map = new ConcurrentHashMap<>();    void calledByMultipleThreads(){        UnsafeStuff mystuff = map.get(Thread.currentThread());        if (mystuff == null){            map.put(Thread.currentThread(),new UnsafeStuff());            return;        }else{            mystuff.modifySomeStuff();        }    }}

The UnsafeStuff objects itself "could be shared" with other threads in the sense that if you'd pass some other thread instead of Thread.currentThread() at runtime to the map's get method, you'd get objects belonging to other threads. But you are choosing not to. This is "usage that is confined to a thread". In other words, the runtime conditions are such that the objects is in effect never shared between different threads.

On the other hand, in the example below the object is automatically confined to a thread, and so to say, the "object itself" is confined to the thread. This is in the sense that it is impossible to obtain reference from other threads no matter what the runtime condition is:

class SomeClass {    void calledByMultipleThreads(){        UnsafeStuff mystuff = new UnsafeStuff();        mystuff.modifySomeStuff();        System.out.println(mystuff.toString());    }}

Here, the UnsafeStuff is allocated within the method and goes out of scope when the method returns.. In other words, the Java spec is ensuring statically that the object is always confined to one thread. So, it is not the runtime condition or the way you use it that is ensuring the confinement, but more the Java spec.

In fact, modern JVM sometimes allocate such objects on stack, unlike the first example (haven't personally checked this, but I don't think at least current JVMs do).

Yet in other words, in the fist example the JVM can't be sure if the object is confined within a thread by just looking inside of calledByMultipleThreads() (who knows what other methods are messing with SomeClass.map). In the latter example, it can.

Edit: But what if I still want to share the object with another thread? Let's say that after thread A finishes with object O, thread B wants to access O. In this case, can O still be confined to B after A is done with it?

I don't think it is called "confined" in this case. When you do this, you are just ensuring that an object is not accessed concurrently. This is how EJB concurrency works. You still have to "safely publish" the shared object in question to the threads.

So when an object is confined to a thread, no other thread can have access to it?

That's what thread confinement means - the object can only EVER be accessed by one thread.

Is that what it means to be confined to a thread?

See above.

How does one keep an object confined to a thread?

The general principle is to not put the reference somewhere that would allow another thread to see it. It is a little bit complicated to enumerate a set of rules that will ensure this, but (for instance) if

• you create a new object, and
• you never assign the object's reference to an instance or class variable, and
• you never call a method that does this for the reference,
• then the object will be thread confined.